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Full text of "Mathematical And Physical Papers - Iii"

304          ON THE  CHANGE  OF  REFRANGIBILITY  OF LIGHT.
crimson than red, arose, no doubt, from the mixture of a little blue or violet light due to false dispersion, and to the scattering which took place at the surface of the glass.
56.    On looking down from above, the places of the more conspicuous bands of absorption were indicated by dark teeth, with their points turned towards the incident light, interrupting the dispersed light.    It is to be understood that the light was transmitted as close as possible to the upper surface, so that the absorption by which these teeth were formed took place before dispersion.    In this way the places of the absorption bands Nos. 1, 2 and 4, were perfectly evident.    No. 3, it will be remembered, was by no means conspicuous.  When the solution is of convenient strength, the absorption is so rapid beyond the bright band No. 5, that the dispersion is confined to a thin stratum close to the surface by which the light enters, and therefore no dark tooth would be seen corresponding to the dark band No. 5.
57.    On following the active light through the spectrum, in the direction of increasing refrangibility, the dispersion was found to commence with a bright but narrow tail of pure red light, which shot right across the vessel.    The light by which this tail, was produced belonged to the more refrangible part of the extreme
j1 J;                           red band which is transmitted by a moderate thickness of the
!' |                          fluid.    The  activity  of   the  incident  light   commenced  almost
!' }f\                           abruptly: the same, it will be remembered, was the case with the
; l|                           absorbing power of the medium.    After the tail of red light came
|                          the intense absorption band No. 1, where the dispersed light was
' j|                          confined to the immediate neighbourhood of the surface by which
* .?                          the active light entered.    At this place a very bright band of
,   '                         dispersed light was visible  on  looking at the  vessel from the
' \j                          outside.  In this part of the spectrum the active and the dispersed
'!i'                          light were both red; but that dispersion was accompanied by a
,                         change of refrangibility was shown by  the  effect of absorbing
|                          media.    Thus the long red tail and the bright band adjacent to
the surface were differently affected by a blue glass, according as
it was held in the first or the second position ; and the bright
i                           band, though much enfeebled, was still plainly visible through a
|                          considerable thickness of the fluid, although a stratum having a
, >(                          thickness of only a very small fraction of an inch was sufficient to